Telephone circuit



Feb. 13, 1940. A. F. JONES 9 TELEPHONE CIRCUIT Filed July 10, 1936 2 Sheets-Sheet 1 ]NVENTOR- Aer/lax Frank/ck da/ves -Feb 13, 1940. A, F, JQNES 2,190,155

TELEPHONE CIRCUIT Filed July 10, 1936 2 Sheets-Sheet 2 Arr/ m FPEDEk/CK don es Patented Feb. 13, 194% UNETED STATES;

TELEPHONE CIRCUIT Arthur Frederick Jones, Coventry, England, assignor to The General Electric Company Limited, Coventry, England Application July 10, 1936, Serial No. 89,917

In Great Britain August 9, 1935 7 Claims.

The present invention relates to {telephone instrument circuits of the type comprising a.

transmitter and a receiver terminating some form of line, and the object of the invention is the production of a line termination of this type which whilst maintaining reasonable efiiciency during reception of speech, and whilst having anti-side-tone properties during transmission, yet has the least possible value of resistance to direct current flowing in the line wires. It will be understood that the transmitter feeding current is supplied by a local battery.

According to the invention, a telephone line termination comprising essentially a transmitter, a receiver and a single transformer, has one winding of the transformer connected directly across the line wires to which the termination is connected so as to form a loop passable by direct current, other windings of the transformer being employed in connection with, the transmitter and receiver such that side-tone is prevented or maintained at a low value, and emciency is ensured during reception and trans-' mission. l I

The invention will be better understood from the following description of several methods of carrying it into effect which are given by way of example and should be'taken in conjunction with the accompanying drawings comprising "Figs. 1-4. Of these Figs. 1-3 Show diagrammatically alternative circuit arrangements of a subscribers telephone instrument incorporating the invention, while Fig. 4 shows the full circuits of a telephone set suitable for connection to an automatic main exchange.

Referring first to Figs. 1, 2 and 3, a transformer having windings A, B and C all on the same core, has one winding C connected to the line wires 2 and 3. and carries any direct current flowing in the line Wires. Another Winding A of the transformer is connected to the transmitter T which is energised by a local battery L. A third winding B connected to one end of winding C through a condenser K acts to suppress side-tone in the receiver it during transmission. A resistance Z is if necessary connected in series or parallel with the receiver in order to regulate the anti-sidetone action of the circuit. In Figs. 1 and 2 the resistance Z acts as a form of line balance and may be varied to suit long or short lines. In Fig. 3 the resistance Z is adapted to act as a potentiometer and variation of the position of the sliding member thereof causes variation in the extent of side-tone suppression provided by the coil B.

This winding C is of low resistance The condenser K is notessentialto the func tioningofthe circuit from the transmission and reception point of view, but is included both in order to prevent the battery L from supplying current to the line Wires 2, 3' and to prevent direct current from the line wires ifsuch is present therein from affecting the subscribers transmission and/or reception.

The method of eliminating or suppressing side tone is as follows:

Potentials are developed across the, impedance of the transmitterT during speech, these potentials being transformed from the coil A into the line circuit by the coil C and also into the receiver circuit; These latter currents are undesirable during transmission, so-that the winding B is inserted in the circuit and is formed of a suitable number of turns and connected in such a manner that during transmission, for a reasonable range of variation of line impedance, the currents produced in the receiver by the actio of the'transmitter are relatively small.-

Thewinding ,C which transmits the speech currents from the transmitterT to the line wires 2, 3is kept at as low a value of ohmic resistance as possible. This enables the resistance of the lines to which the circuit is connected to ,be increased above that possible when a normal type of telephone circuit is employed, so thatlonger lines, or Wires having smaller diameter may be used. The dimensions of the coils of the transformer are so chosen that no appreciable diiference is produced in the efiiciency of transmission or reception ascompared with similar circuits.

Referring now to Fig. 4, a circuit is shown suitably adapted for use in connection with an automatic main exchange. The line circuit from Wire 2 to 3 is normally broken at the contact H2 which is closed when the subscriber lifts his re ceiver from the hook switch. This completes a circuit through the coil C and dial impulse springs M to the main exchange. Automatic selecting switches are there connected to the line wires and are positioned in known manner by the operation of the springs M. Further contacts Hi on the hook switch close the local circuit from the batteryL through the transmitter T and the coil A, and also complete the receiver circuit through the coil B. Whilst the dial is off normal, contacts N i,' N2 short circuit the winding A and the receiver B This leaves winding B and the condenser K as shown across the dial springs, thus reducing sparking at the contacts, and also prevents the subscriber from hearing noises in the receiver caused by impulsing.

The subscriber is called by application to the line wires 2, 3 of ringing current by main exchange apparatus, this operating the bell G through the bell G through the condenser K, When the subscriber closes the line loop by removing his receiver the ringing is tripped in known manner.

The transformer employed may have a number of forms, but the following details relate to a typical case which has been found suitable.

Core. iron wire, open ended.

Winding A.-' 200 turns of .014 diameter enamelled and single silk covered copper wire, resistance 2.5 ohms.

Winding B.-l800 turns of .0076" single silk covered copper wire, resistance 56 ohms.

Winding C.3000 turnsv of .01 diameter enamelled and single silk covered copper wire, resistance 40 ohms.

All windings are laid on in the same direction. The inner ends of the coils are the ones designatedwith odd figures.

I claim:

1. In a substation telephone circuit, a line, a transformer having three windings, a receiver connected in series with a first and a second winding of said transformer and the conductors of said line, a third winding of said transformer bridging said line conductors to provide a low resistance direct current path for current flowing in said line, a transmitter, and a local battery connected in series with said transmitter and shunting said first winding of said transformer to provide a transmission circuit.

.; 2. In a substation telephone circuit, a line, a

transformer having three windings, a first wind.-

ing of said transformer directly connected across the conductors of said line to form a direct current path, a transmitter, a direct current source,

,a second winding of said transformer connected "in series with said transmitter and said current source to provide a local circuit in which voice currents are generated during transmission, a receiver, a variable resistance connected to one of the line conductors of said line and said transmitter circuit, and a third winding of said transformer and said receiver connected to said transmitter and the sliding contact of said variable resistance for adjusting side tone suppression.

3. In a substation telephone circuit, a line, a transformer having three windings, a switchhook and contacts therefor, impulse springs, a first winding of said transformer included in a series circuit with said impulse springs, certain ofsaid switchhook contacts and said line; a ringer, a condenser and said ringer connected in parallel with respect to said series circuit, a transmitter, a local battery, a second winding of said transformer included in a series loop circuit with said transmitter and said local battery, a receiver, a third winding of said transformer and said receiver connected in series with said series loop circuit and bridging said ringer, and other contacts of said switchhook for completing said series loop circuit during conversation and for disconnecting said series loop circuit when said receiver is placed on said switchhook.

4. In a telephone circuit, a transformer, a line, a first winding of said transformer directly connected across the conductors of said line to permit the passage of direct current, a transmitter, a direct current source, a second winding of said transformer connected in series with said transmitter and said current source forming an energizing circuit for said transmitter, a receiver, a resistance in series with said receiver connecting one end of said first and second transformer windings, a condenser, and a third winding of said transformer in series with said condenser connecting the other end of the first and second transformer windings to suppress side-tone in said receiver during transmission.

5. In a substation telephone comprising a transmitter, a receiver, a resistance, a condenser, a transformer having three windings, a local battery, and a line; a closed local circuit including one winding of said transformer, said transmitter, and said battery; a second winding of said transformer bridging the conductors of said line so that direct current from the line passes therethrough, a shunting circuit for said receiver including a third winding of said transformer, and a bridge across said line including said condenser, said resistance, said shunting circuit and said closed circuit.

6. In a substation telephone, a transmitter, a receiver, a resistance, a condenser, a transformer having three windings, a local battery, and a line; a closed local circuit including one winding of said transformer, said transmitter, and said battery; a second winding of said transformer bridging the conductors of said line so that direct current from the line passes therethrough, a loop circuit including said receiver, said resistance and a third winding of said transformer, and a bridge across said line including said condenser, said loop circuit, and said closed circuit.

7. In a substation telephone circuit, a transformer, a receiver, a line and a condenser, a loop including the line and one winding of the transformer passable by direct current, another loop including two other windings of the transformer and said condenser, a transmitter connected in bridge of one of said other windings, said receiver connected to said loops in such a manner as to reduce side tone to a very low value in said receiver.

ARTHUR FREDERICK JONES. 

